Information and Advisory Note Number 31 Back to menu
RVHD is a viral disease of the rabbit Oryctolagus cuniculus. It was first
described in domestic rabbits in China and entered Europe through the south
west. It affects both wild and domestic animals and has been recorded in
Europe's domestic stock since 1988.
The disease was first recorded in Britain in April 1992. Initially the majority
of cases were reported from the southern counties of England, particularly the
south west, with isolated cases elsewhere in Britain. However, this pattern has
altered during the past couple of years and a total of 512 cases were reported
across Scotland, England and Wales in 1995, of which four cases were reported
from Scotland. Of this total, 8 cases were reported from animals in the wild.
RVHD is a notifiable disease and all suspected cases in Scotland must be
reported to the Scottish Office (see 3.5). These records are then collated for
the UK by the Ministry of Agriculture, Fisheries and Food.
2.1 Historical origin
Rabbits were introduced to Britain by the Normans, around the 12th or 13th
century. At that stage they were not technically wild but were initially
confined to managed warrens, primarily on islands and in coastal areas. The
trade in rabbit meat was well established by the 14th century and reached its
peak around 1800 (Sheail 1971). However, the economic value of the species had
declined by the mid-
19th century, and measures were begun to control rabbits to reduce agricultural
damage.
Rabbits were widespread in Britain by the 16th century although their spread in
Scotland was more gradual. While apparently abundant around Edinburgh at this
time, they were otherwise primarily restricted to a few islands and coastal
habitats, where they often constituted the main source of income. They were
rarely recorded north of the Tay and Clyde valleys. The spread of rabbits
throughout Scotland is largely attributed to introductions throughout the
Highlands in the 18th and 19th centuries, aided by improved agricultural
practices.
The most marked reduction in rabbit populations in modem times came as a result
of the myxomatosis virus. This began in Edenbridge, Kent, in October 1953 and is
believed to have come from populations in France, possibly introduced by farmers
as an efficient form of pest control (Tapper 1992). The first wave of this
disease killed 99% of all rabbits infected. Although this disease is still
encountered in British populations, rabbits have developed an immunity and
genetic resistance is now well-established. Less virulent strains of the virus
have also arisen, with attenuation of the viral strain first being noticed in
1955.
2.2 The role of rabbits The rabbit is a keystone species for the maintenance of
many habitats, for example chalk downs in southern England and Machair. They are
an also important source of prey for many predatory species, for example
buzzards Buteo buteo, foxes Vulpes vulpes and some mustelids. It is a highly
successful species, is generally resistant to disease and parasites, is
adaptable under adverse conditions and has a high reproductive potential
(Thompson 1994). It is now estimated that there are 37.5 million rabbits in
Britain of which there are 9.5 million in Scotland (Harris et al. 1995).
2.3 Habitat
Rabbits are typically found in short "grass habitats on machairs, dry heaths and
agricultural land. They are able to graze these areas closer to the ground than
sheep and are well-known catholic feeders which produce the so-called 'disclimax
community' of open grassland. Rabbits graze these areas all the year round but
the impacts are most severe in the winter. As a consequence of its feeding
habits, the species is considered to inflict more damage to agricultural crops
than any other pest species in Great Britain (Trout et al 1992).
2.4 Feeding behaviour
Foraging behaviour is mainly crepuscular (i.e. active morning and evening)
although animals may be seen grazing close to burrows in undisturbed areas
during the day. Rabbits eat a wide variety of plant species but favour young
succulent leaves and shoots and more nutritious species such as the Festuca
grasses. The role of rabbits as agricultural pests due to their feeding
behaviour was well known prior to the 1950s, but the extent to which they
influenced other species and habitats was recognised only when myxomatosis
reduced the population in the mid-late 1950s (Thompson 1994).
2.5 Reproduction
One of the main reasons for the success of this species is its reproductive
potential. The breeding season is mainly from January to June with sporadic
breeding continuing up to August. A succession of litters may be produced at
minimum intervals of 30 days, with an average litter size of between 3 and 7
young. The rabbit is an induced ovulator and postpartum conception is normal,
with females usually pregnant again within 24 hours of the previous birth.
However, subordinate females are usually less successful and have a shorter
breeding season and higher rate of loss of embryos than older, dominant
individuals.
What is RVHD?
RVHD is a single-stranded RNA, round
icosahedral virus, with no envelope. It can
remain viable in temperatures up to 60°C and is thought to survive in the
environment for several months. Contrary to its name, the resultant disease
agglutinates erythrocytes in the blood, causing it to clot. Characteristic
pathological changes include punctate haemorrhage In the respiratory and
digestive systems, the spleen, cardiac muscles and, occasionally, the kidneys
(Rogers et al. 1994).
3.1 What are the symptoms?
Symptoms are not universal and may include
loss of appetite, lack of co-ordination,
respiratory difficulties and discharge of blood
from orifices (including nose-bleeds).
However, animals generally show few external
symptoms of the disease and it is much more
likely that an apparently healthy-looking
animal will just 'drop dead'. The incubation
period is between 24 and 72 hours with death
from several hours to one or two days later.
RVHD is a very difficult disease to diagnose in
the wild. There is a marked difference
between the apparently healthy-looking
corpses of animals dying of RVHD and those
dying of myxomatosis.
3.2 Transmission
The disease is highly contagious and is spread by direct contact with infected
animals, by contaminated food, water, bedding and by mechanical transmission.
Although humans may carry the disease on clothing, etc., RVHD is specific to
rabbits with no evidence of any threat to human health even if infected rabbits
are eaten. There is limited evidence of the disease being transmitted to hares (Lepus)
under experimental conditions but there is no evidence of this occurring in the
wild. It is related to, but is different from, European Hare Syndrome which has
been present in the UK for some time.
3.3 The European experience
The disease was first recorded from Europe in 1988. Within a few years it had
caused large mortality in domestic and wild rabbits in Italy, Germany and
Austria. Spain was one of the worst affected countries where, initially, the
disease was detected at two locations: one in the north and one in the south
east of the country. However, in less than two years the virus had spread
throughout the whole of the country, with estimated mortality levels of up to
90% of the rabbit population. At some sites, the disease progressed so rapidly
that rabbits were dying faster than the predator population could remove the
carcasses from the ground (Rogers et at. 1994).
Recent studies in the predator-rich area of the Mediterranean suggest that
rabbits may be developing a genetic resistance to the disease. As such there is
a potential for pockets of resistance to develop and predictions are that an
equilibrium will develop between the rabbit and the disease as per the
myxomatosis experience (Rogers et al. 1994).
3.4 What are the implications of the disease in Scotland?
Parallels with myxomatosis are clear, although current indications are that RVHD
will not result in the dramatic population decline seen with the former.
However, RVHD does have the potential to reduce significantly rabbit populations
both locally and nationally. This has obvious implications for a number of
ecological processes. Such effects have been documented in the literature
(Rogers et al, 1994; Thompson 1994; Sumption and Flowerdew 1985). The most
significant threat to the natural heritage in Scotland is through the reduction
in prey availability for raptor and predator species. The European experience
suggests that there is no chance of containing the disease in the wild rabbit
population.
3.5 What is being done about the disease? RVHD is a notifiable disease in
Scotland by virtue of Schedule 1 of the Specified Diseases Notification Order
1991, as amended. Consequently, any person who has in their possession a
diseased or suspected diseased animal or carcass, must inform the authorities of
the fact. This may be done through the local Government Animal Health Office, or
through a local Police Station, which will then forward the report to the
Scottish Office. If a case is reported from a commercial rabbitry, veterinary
inspectors are currently instructed to place a Movement of Animals
(Restrictions) Order 1990 on the premises to prohibit the transfer of animals on
to or off the premises, except under licence. This is left in place until the
veterinary inspector is satisfied that the disease no longer exists on the
premises, or that keeping it in place is no longer necessary for the purpose of
the Order. Compulsory slaughter is not required and compensation is not paid.
An injectable vaccine was developed for the protection of domestic stock in
Europe and this was developed into two inactive vaccines for the protection of
stock in the UK. These were granted one-year provisional licences through the
Veterinary Products Committee of MAFF. It is anticipated that they will be
retained for use in future years. However, a
programme of vaccination for the wild population would pose many problems and
the potential for containing the disease this way is slight.
Investigations to date suggest that there is a high level of seropositivity in
the British rabbit population although the reasons for this are unknown. It is
hypothesised that British rabbits may have been exposed to a similar, but
non-lethal, virus in the past. This has not yet been confirmed to equate with
natural immunity. In add-on, there is a suggestion that young rabbits are immune
to the virus for the first two months of life. This will have obvious
implications for the timing of an outbreak.
Despite the virulence of the disease in the rabbit population, there are
currently proposals from Government to de-classify the notifiable status of the
disease. This is being considered on the grounds that reports of the disease are
increasing, that a commercial vaccine is now available (and veterinary advice is
to vaccinate domestic rabbits) and the fact that there is no commercial benefit
to be gained from the current statutory control measures. However, in view of
the difficulty of identifying the disease, removal of the reporting requirement
will increase the difficulty of tracking the disease in the wild population.
There is currently no alternative method of monitoring the spread of the disease
in Britain. Consultation on this recommendation is in the early stages.
Initial predictions of the impact of RVHD on the British rabbit population were
based on experience from Continental Europe. This predicted losses of up to 90%
of the total population. Subsequent research has shown that British animals have
a generally higher level of seropositivity to the disease and, therefore, may
not be subject to the high level of decline experienced elsewhere.
Victims of this disease do not exhibit obvious external symptoms, hence
detection of RVHD in the wild rabbit population is extremely difficult. The most
likely method of detecting the effects will be through monitoring to detect
increases in rabbit mortality not attributable to natural processes. This will
be of greatest value at sensitive sites where the rabbit population is key to
the balance of the ecosystem. Given the proposal to declassify
the disease, some level of monitoring becomes even more important in these
areas. The first step to the implementation of this approach is the
identification of rabbit-dependent sites. This work is currently being done in
RASD.
From available evidence it would seem that it is not possible to stop the spread
of this disease other than through the use of vaccine, which is not deemed
practicable in a wild population. Consequently, where the effects of the disease
are recognised to be detrimental, it may be necessary to compensate artificially
for the reduction in grazing pressure or loss of prey species. Such actions will
need to be identified in a contingency plan prepared for sensitive sites.
It is also important to ensure that incidents of RVHD continue to be recorded.
If the current proposal to remove the reporting requirement is approved, this
responsibility will no longer fall to the Government. Under these circumstances,
it will be important to ensure that reports of suspected cases are known to SNH.
Staff encountering the disease in their area are therefore encouraged to report
the location and date of confirmed, or suspected, cases to the RASD contact
given in Section 6.
Harris, S., Morris, P., Wray, S. and Yalden, D. 1995. A review of British
mammals Joint Nature Conservation Committee, Peterborough.
Rogers, P.M., Arthur, CP. and Sorigeur, R.C 1994. The rabbit in Continental
Europe. In: The European rabbit: the history and biology of a successful
coloniser, ed. by H.V. Thompson, & CM. King. Oxford University Press, Oxford, pp
22-63.
Sheail, J. 1971. Rabbits and their history. David and Charles, Newton Abbot.
Sumption, K.J. and Flowerdew, J.R. 1985. The ecological effects of the decline
of rabbits {Oryctolagus cuniculus) due to myxomatosis. Mammal Review 15:151-186.
Tapper, S. C 1992. Game heritage.
Game Conservancy Trust, Fordingbridge.
Thompson, H. V. 1994. The rabbit in Britain. In* 77je European rabbit: the
history and biology of a successful coloniser, ed. by H.V.
Thompson, & CM. King. Oxford University Press, Oxford, pp 64-107.
Trout, R.C., Ross, J., Tittensor, A.M. and Fox, A.P. 1992. The effect on a
British wild rabbit population (Oryctolagus cuniculus) of manipulating
myxomatosis. Journal of Applied Ecology 29:679-686.
Dr Main Cooper
Species Conservation Officer
International and Biodiversity Branch
Scottish Natural Heritage
2 Anderson Place
Edinburgh EH6 5NP
Tel: 0131-447 4784